Abstract: Objective To evaluate the relationship between the root and alveolar ridge crest of the first and second maxillary molars and maxillary sinus floor by cone-beam computed tomography (CBCT), and to provide reliable reference for clinical diagnosis and treatment. Methods One hundred and eighty cases were selected from the CBCT database of the Affiliated Stomatological Hospital, School of Stomatology, Dalian Medical University, and divided into three groups (6-12 years old, 13-18 years old and 19-35 years old), with 60 cases in each group. CBCT was used to measure distance between each root apex of the first and second maxillary molars and maxillary sinus floor, distance between the alveolar ridge crest and the maxillary sinus floor. Comparison was conducted between different sides and genders respectively. Classification of the vertical relationship between each root of the first and second maxillary molars and the maxillary sinus floor was recorded and statistical analysis was carried out. Results In 6-12 age group, average distance between the mesiobuccal root of the first maxillary molar and the maxillary sinus floor ((1.21±3.09) mm) was the shortest among the maxillary roots. In 13-18 age group, the apices were all in the maxillary sinus. In 19-35 age group, the palatal root was ((0.41±2.73) mm). In 13-18 and 19-35 age groups, average distances between the mesiobuccal root of the second maxillary molar and the maxillary sinus floor (0.36±2.37) mm and (0.43±2.21) mm both were the shortest among the maxillary roots. The average distance between the alveolar ridge crest and the maxillary sinus floor decreased with ages (P<0.05). In the horizontal relationship, in all age groups, the maxillary sinus was most frequently observed at the level of root apices to root furcation in 13-18 age group and 19-35 age group. In the vertical relationship, in all age groups, the most common condition was that all three root apices were not in contact with the maxillary sinus floor of the first molar in 6-12 age group and 19-35 age group, and the second molar in 13-18 age group. Conclusion The first and second maxillary molars are closely related to maxillary sinus floor. It is suggested that CBCT should be applied before clinical treatment, which has positive significance for analyzing anatomical position of the posterior maxillary region and reducing incidence of maxillary sinus complications.

Key words: cone-beam computed tomography, maxillary sinus floor, root apices of maxillary molars, alveolar bone